Various methods of detecting particles in air are known. One method involves projecting a beam of light across a detection chamber in which an air sample is contained and measuring the amount of light scattered at a particular scattering angle from the beam. Such particle detectors can be aspirating, in that they actively draw air into them, or alternatively they may rely on natural air currents to move air into the detection chamber.
It is known that the angular scattering properties of particles are dependent on the wavelength of incident light relative to the particle size. Accordingly, smoke and particle detectors have been made using multiple scattering angles and/or multiple wavelengths to detect particles of a predetermined size of interest. For example, UV light is scattered relatively strongly by small particles, such as smoke, but infrared light is scattered less by such particles. On the other hand UV and IR light will be equally sensitive to variations in received light intensity that are caused by factors such as drift in the system, soiling of the optics of the system or the introduction of large particles such as dust into the detection chamber.
With such multiple wavelength or scattering angle particle detection systems there is a need to be able to accurately determine whether the received signals at the multiple scattering angles or wavelengths are caused by particles of interest or nuisance particles such as dust. It is also possible to use this wavelength sensitivity or angular sensitivity of light scattering to track the nature of group of particles over time, e.g. to track the development of a fire as the smoke particle size distribution changes.
Accordingly, it is an object of an aspect of the present invention to provide means for determining whether particles of interest have entered a detection chamber using more than one wavelength of electromagnetic radiation.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.